Lab 7 - EE 420L 

Author: Dane Gentry

Email: gentryd2@unlv.nevada.edu

April 6, 2016

   

Design of an Audio Amplifier

       

Click on any picture for its full size!

   

Pre-lab work

Lab Description

Lab Requirements

Design an audio amplifier (frequency range from roughly 100 Hz to 20 kHz) assuming that you can use as many resistors, ZVN3306A transistors, and ZVP3306A transistors as you need along with only one 10 uF capacitor and one 100 uF capacitor. Assume that the supply voltage is 10 V, the input is an audio signal from an MP3 player (and so your amplifier should have at least a few kiloohms input resistance), and the output of your design is connected to an 8-ohm speaker (so, ideally, the output resistance of your amplifier is less than 1 ohm).


For the following questions and experiments assume VCC+ = +10 Volts

Below is a comparison between driving a speaker without (red, Vout1) and with (black, Vout2) an audio amplifier. The source resistance is 10k meaning that the source can supply 1 V (blue, Vs) at 100 uA maximum. The simulation files used to generate this figure are found in lab7_sims.zip.
                                     Schematic                                                                                                                               Waveform Simulation
http://cmosedu.com/jbaker/courses/ee420L/s16/students/gentryd2/lab7/SS's/Schem's/Baker%20given%20Sim.JPG    http://cmosedu.com/jbaker/courses/ee420L/s16/students/gentryd2/lab7/SS's/Schem's/Baker%20given.JPG
         
Improved Audio Amplifier Design
Our audio amplier design utilizes the push pulll amplifier from Lab 6. This topology incorporates the same principle as a voltage divider such that the output voltage is large when the output resistance (resistor from output to ground) is large while the input resistance (resistor from input to output) should be small, and vice versa. The output resitor in our audio amplifier is the 8 ohm speaker load that the amplifier will be driving. The input signal will be amplified at the output to produce louder audio out of the speaker. Our audio amplifier has a calculated gain of Vout/Vin = Rf*(gmp+gmn) without the speaker load. In order to better match the speaker load, a smaller output resistance was provided to our amplifier by adding a second stage source follower which decreases the output impedance while the gain is close to 1.


                        Schematic                                                                                                                    Waveform Simulation
http://cmosedu.com/jbaker/courses/ee420L/s16/students/gentryd2/lab7/SS's/Sim's/Schem.JPG    http://cmosedu.com/jbaker/courses/ee420L/s16/students/gentryd2/lab7/SS's/Sim's/Sim.JPG
         
                       Input Resistance                                           Output Resistance
http://cmosedu.com/jbaker/courses/ee420L/s16/students/gentryd2/lab7/SS's/Sim's/Input.JPG  http://cmosedu.com/jbaker/courses/ee420L/s16/students/gentryd2/lab7/SS's/Sim's/Output.JPG

The output resistance is simulated to be roughly 10k ohms while the input resistance is simlated to be 8 ohms, which makes sense because we are using an 8 ohm speaker load for the amplifier to drive.
     
               Power Consumption Simulation                        Power Consumption - Voltage/Current Measurement
http://cmosedu.com/jbaker/courses/ee420L/s16/students/gentryd2/lab7/SS's/Sim's/Power%20Sim.JPG    http://cmosedu.com/jbaker/courses/ee420L/s16/students/gentryd2/lab7/SS's/Sim's/Power.JPG
     
Power Consumption Calculation:
The power consumption of the improved design of the audio amplifier circuit was simulated in LTSpice to provide a power consumption that swings between 4.3W (Watts) and 5.12W. The power consumption was then calculated by measuring the voltage (VDD) and drain current of the circuit. The voltage was measured to be 10V while the current was measured to be 282mA. This gives a power consumption of 2.8W b/c 10*280mA = 2.8W.
   
Experimental Results                                                           
http://cmosedu.com/jbaker/courses/ee420L/s16/students/gentryd2/lab7/SS's/Sim's/Osc.JPG

Simulations, and experimental results all agree.
     

Lab Conclusion

This lab demonstrated the design, building, and testing of an audio amplifier utilizing the push-pull amplifier topology. The output of our 2 stage amplifier is an improvement upon the 1 stage amplifier, but the audio signal was both amplified and reduced due to mismatching of impedances since the output of a source follower is significantly smaller than that of the push pull amplifier while our speaker load (8 ohms) is very small, so the output resistance is not small enough to drive the speaker. Ideallly, the output resistance would be smaller than the 8 ohm speaker loud so that the output voltage would be greater. The experiments in this lab provided excellent experience in how to design, build, and utilize an audio amplifier, and all experiments in this lab were performed with little difficulty and few encountered problems.

   

   

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